It is common practice to bend pipe and shape the ends of pipes so that they can be longitudinally connected. For example, in manufacturing automobile exhaust pipes, several bends must be made on the pipe. Moreover, it is often desirable to flare one end opening of the pipe or to provide an increased internal diameter portion along one end for a short distance for receiving the end of another pipe. All of these operations are normally carried out by hydraulic rams and shaping dies.
In the operation of flaring the end of a pipe or simply providing for an increased diameter portion along a short length at one end, an expansion die is normally utilized coupled to the end of a hydraulic ram. The pipe itself is clamped in a fixed position and the die then urged into the end of the pipe by the hydraulic ram. In a similar manner, the die is removed by simply reversing the direction of the hydraulic ram to pull the die free of the end of the shaped pipe. Both this pushing and pulling force in an axial direction of the pipe requires a very secure clamping of the pipe in position.
The type of clamps utilized in the foregoing operation most often take the form of an over-center clamp formed of lower and upper clamp members cammed together into tight engagement with a longitudinal portion of the pipe by an over-center lever and cam action. The clamping force exerted by such clamps is necessarily of a constant or fixed value determined by the camming force applied by the over-centering lever. If this fixed gripping force is not enough to hold the pipe in an end-forming operation, the pipe will simply slip longitudinally in the clamp. On the other hand, the applied force may be more than necessary to properly hold the pipe axially stationary with possible damage to the exterior portion of the pipe being clamped.